1. Field of the Invention
This invention relates to an optical recording and/or reading apparatus, and, in particular, to such an optical recording and/or reading apparatus which is used in an optical type disc information recording/reading system in which information is optically recorded onto or read off of a recording disc such as an optical video disc. More specifically, the present invention is directed to a structure for properly positioning an objective lens forming a part of such a non-contact type optical recording/reading apparatus relative to the recording surface, whereby the positioning of the lens includes proper focusing and tracking.
2. Description of the Prior Art
In a contactless recording/reading system using a laser beam, no recording grooves are formed in the surface of a recording disc, and, instead, as shown in FIG. 1, egg-shaped pits 2 are formed in the surface of a recording disc 1 and arranged circumferentially to form recording tracks. Typically, such pits 2 are only 0.1 microns deep, which is 1/8 of the wave length of the laser beam used for scanning the recording tracks. When these pits 2 are scanned by the laser beam, the reflecting light from the pits 2 is shifted in phase as compared with the reflecting light from the remaining portion of the recording surface. In this manner, the scanning laser beam may be modulated depending upon presence or absence of such a pit 2. Thus, in the read mode, such a phase shift in the laser beam is detected to reconstruct the information recorded in the recording disc 1. Accordingly, image information and sound information are all recorded in the form of pits 2 by varying their length and spacing. Typical sizes of such pits 2 are also indicated in FIG. 1.
In the case of writing information in or reading the recorded information from the recording disc 1, consideration must be given to two important factors: (1) tracking control for preventing the scanning beam deviating from the current recording track and (2) focusing control for keeping the scanning beam completely focused on the surface of the recording disc 1.
FIG. 2 shows a typical prior art optical recording/reading apparatus which, generally, includes a tracking control actuator 3 and a focusing control actuator 4. As the scanning light beam L.sub.b such as laser beam is led toward the actuator 3 as indicated by the arrows, it is reflected toward the focusing control actuator 4 where the beam passes through an objective lens 4a and is focused onto the recording surface 1.sub.f of the disc 1 in the form of a minute spot. The light beam L.sub.b is modulated in intensity at the recording surface 1.sub.f, and the thus modulated light beam is then reflected to travel the same optical path in the reversed direction, and the reflected light beam is then detected by an appropriate detector. Accordingly, such a light beam thus detected may then by demodulated to obtain video and/or audio signals.
The focusing control actuator 4 is similar in structure to the voice coil driving section of a loudspeaker. As shown, the actuator 4 includes a magnetic circuit 4b and a voice coil 4c. Since the structure and operation of a loudspeaker voice coil is well known in the art, it will be briefly described as it applies to the structure and operation of the focusing control actuator 4.
The magnet circuit 4b is comprised of a permanent magnet and soft-iron members and it is so shaped to define a gap in which the voice coil 4c is movably provided. In the gap is formed a high density magnetic flux which extends in the direction normal to the wires forming the coil 4c. The voice coil 4c is supported by a flexible member 4d such that it may move upward or downward in the vertical gap which is parallel with the optical path. Integrally provided with the voice coil 4c by means of an appropriate holder is the objective lens 4a which is thus moved upward or downward together with the voice coil 4c. When the voice coil 4c is supplied by a current which is proportional to the focusing error signal generated by a focusing error detecting system (not shown), the electromagnetic interaction between the current thus supplied and the magnetic field applied to the coil 4c causes the voice coil 4c to move in parallel with the optical path, either upward or downward depending upon the direction of the current, thereby maintaining the lens 4a in the in-focus condition.
The tracking control actuator 3 used in the apparatus of FIG. 2 is of the so-called oscillating mirror type. Stated more in detail, oscillatably mounted on a base 3b through an elastic member 3a of rubber or the like is a mirror 3c, on both ends of which are mounted magnets 3d and 3e. When current is flowed through the coil 3f, the magnets 3d and 3e are either repelled or attracted, so that the mirror 3c becomes oscillated around the elastic member 3a, thereby varying the incident angle of the light beam L.sub.b entering into the lens 4a.
In this manner, in accordance with the prior art, two separate actuators are provided, one for tracking control and the other for focusing control, as arranged spaced apart from each other along the optical path. For this reason, the prior art apparatus including the two separate actuators 3 and 4 tends to be bulky and difficult to manufacture partly because the two actuators must be aligned accurately during manufacture. Making the whole apparatus compact in size is desirable because it then allows to shorten the distance between the objective lens 4a and the light receiving element such as the recording surface 1f, which then contributes to increase the detection limit of the light receiving element. In this respect, more detailed explanation will be given hereinbelow.
In a typical prior art in-focus detecting system as shown in FIG. 3, there are provided a quarter wave plate 5, a beam splitter 6, a deflecting prism 7, a pair of light-receiving elements 8a, 8b, a coupling lens 9 and a light source 10 such as a laser diode. In the illustrated system, a focusing error is detected by the difference in amount of light received by the pair of light-receiving elements 8a and 8b which are arranged symmetrically with respect to the optical axis. Assuming that the disc 1 is located at the in-focus position, the reflecting light from the disc 1 is led as parallel light to the light-receiving elements 8a and 8b, as shown in FIG. 3. On the contrary, if the disc 1 is located far away from the focal point of the objective lens 4a, as shown by the two-dotted line in FIG. 3, then the reflecting light from the disc 1 comes to be focused behind the light-receiving elements 8a and 8b, as shown in FIG. 4; on the other hand, if the disc 1 is brought closer to the lens 4a than the local point, the reflecting light comes to be focused in front of the light-receiving elements 8a and 8b, as shown in FIG. 5. In either case, when the disc 1 tends to be located further away from the focal point of the lens 4a shown in FIG. 3 (cf. FIG. 4), the light fluxes on both sides of the optical axis maintain the same positional relation with respect to the paired light-receiving elements 8a and 8b; however, if the disc 1 tends to move closer to the lens 4a beyond the focal point, the positional relation of the light fluxes on both sides of the optical axis with respect to the light-receiving elements 8a and 8b becomes reversed (cf. FIGS. 4 and 5), so that focusing error cannot be detected by using the difference in amount of light received by the light-receiving elements 8a and 8b.
With foregoing in mind, in order to properly detect focusing error with the system such as shown in FIG. 3, it must be so structured that the light beam L.sub.b is always focused behind the light-receiving elements 8a and 8b. Under the circumstances, defining the amount of shift of the disc 1 in the direction of the optical path which satisfies the above-described condition, or the detection limit of the light-receiving elements 8a and 8b by e, it may be expressed by the following equation. EQU e=f.sup.2 /2(1-f)
where,
f is the focal distance of the objective lens, and 1 is the equivalent optical path length between the objective lens and the light-receiving elements.
As may be understood from the above equation, the value of e may be increased by making the optical path length difference (1-f) smaller, indicating an increase in detection limit of the light-receiving elements.